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File:Black Widow 11-06.jpg
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Order: Araneae
Family: Theridiidae
Genus: Latrodectus
Charles Athanase Walckenaer, 1805
Species

Approx. 31, see article

Latrodectus is a genus of spider, in the family Theridiidae, that contains 31 recognized species. The common name widow spiders is sometimes applied to members of the genus due to the supposed habit of the female of eating the male after mating, although the males of most species are not usually eaten after mating, and can even go on to fertilize other females.[1] The black widow spiders are perhaps the best-known members of the genus. The injection of neurotoxic venom latrotoxin from these species is a comparatively dangerous spider bite, resulting in the condition latrodectism, named for the genus. The female black widow's bite is particularly harmful to humans because of its unusually large venom glands.

Description

Not all adult female black widows exhibit the red hourglass on their abdomen—some may have a pair of red spots or have no marking at all, but any markings that are present are bright red. Adult male black widows are a quarter the size of the female, and are usually gray or brown rather than black and red; while they may sometimes have an hourglass marking on their abdomen, it is usually yellow or white, not red. The bite of a male black widow is not considered dangerous to humans; it is the bite of the adult female black widow from her much larger venom sacs that has given this spider its dangerous reputation. While there is great variation in specifics by species and by gender, any spider exhibiting a red hourglass on the abdomen and having a shiny black body is an adult female black widow.

Spiders of the genus Steatoda (also of the Theridiidae family) are often mistaken for widow spiders, and are known as "false widow spiders"; they are significantly less harmful to humans.

In common with other members of the Theridiidae family, the widow spiders construct a web of irregular, tangled, sticky silken fibers. The spider very frequently hangs upside down near the center of its web and waits for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to bite it and wrap it in silk. If the spider perceives a threat, it will quickly let itself down to the ground on a safety line of silk. As other web-weavers, these spiders have very poor eyesight and depend on vibrations reaching them through their webs to find trapped prey or warn them of larger threats. While there are some more aggressive species, most are not; many injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched. Some bites are thought to result from a spider mistaking a finger thrust into its web for its normal prey[citation needed], or in cases where a female is protecting an egg sac, but ordinarily intrusion by any large creature will cause these spiders to flee.

Strength of Latrodectus silk

Silk from L. hesperus spiders is reputed to be particularly strong compared with the silk of other spiders.[2][3] However, the results of a study by Blackledge, et al. do not confirm this.[4]

The ultimate strength and other physical properties of Latrodectus hesperus (western black widow) silk were found to be similar to the properties of silk from orb-weaving spiders that had been tested in other studies. The ultimate strength for the three kinds of silk measured in the Blackledge study was about 1000 MPa. The ultimate strength reported in a previous study for Nephila edulis was 1290 MPa ± 160 MPa.[5] The tensile strength of spider silk is comparable to that of steel wire of the same thickness;[6] as silk is about six times less dense than steel,[7] it is correspondingly stronger than the same weight of steel.

See the spider silk and Tensile strength articles for details about the strength of spider silk in general.

Species

The southern black widow, as well as the closely related western and northern species which were previously considered the same species, has a prominent red hourglass figure on the underside of its abdomen. Many of the other widow spiders have red patterns on a glossy black or dark background, which serve as a warning. Spiders which are found in multiple regions are listed in their predominant native habitat.

Widow spiders can be found on every continent of the world except Antarctica. In North America, the black widows commonly known as southern (Latrodectus mactans), western (Latrodectus hesperus), and northern (Latrodectus variolus) can be found in the United States, as can the "gray" or "brown widow spiders" (Latrodectus geometricus) and the "red widow spiders" (Latrodectus bishopi) (Preston-Malfham, 1998). The single species occurring in Australia is commonly called the redback (Latrodectus hasselti). African species of this genus are sometimes known as button spiders.

America

File:Blackwidow eggsac silk.jpg
File:Brown widow spider Latrodectus geometricus underside.jpg

The following widow spiders are indigenous to North America:

The following are indigenous to Central and South America:

Europe, North Africa, the Middle East, and western Asia

File:Latrodectus tredecimguttatus female.jpg

The following widows indigenous to the Mediterranean region, as well as in western Asia:

Sub-Saharan Africa and Madagascar

File:Latrodectus.elegans.male.-.tanikawa.jpg

South, East, and Southeast Asia

Australia and Oceania

Worldwide

References

  1. Breene, R. G. and M. H. Sweet (1985). Evidence of insemination of multiple females by the male Black Widow Spider, Latrodectus mactans (Araneae, Theridiidae). The Journal of Arachnology 13 (3): 331–335.PDF
  2. Biologists Unravel The Genetic Secrets Of Black Widow Spider Silk. Science Daily. URL accessed on 2009-01-23.
  3. Piquepaille, Roland The genetic secrets of the black widow spider. ZDnet. URL accessed on 2009-01-27.
  4. Blackledge, et al., Todd Quasistatic and continuous dynamic characterization of the mechanical properties of silk from the cobweb of the black widow spider Latrodectus hesperus. The Company of Biologists. URL accessed on 2009-01-23.
  5. Blackledge, et al., Todd Quasistatic and continuous dynamic characterization of the mechanical properties of silk from the cobweb of the black widow spider Latrodectus hesperus, table 1. The Company of Biologists. URL accessed on 2009-01-23.
  6. Astm a36. OnlineMetals.com. URL accessed on 2009-01-25.
  7. Elices et al., Manuel Finding Inspiration in Argiope Trifasciata Spider Silk Fibers id=vIhvSQLhhMEC&pg=PA616&lpg=PA616&dq=1085+steel+wire+ultimate+strength&source=web&ots=ET0u9ifd6L&sig=B1HptQCs4DNl4V7h28lDZnZYNcA&hl=en&sa=X&oi=book_result&resnum=6&ct=result#PPA616,M1. JOM, February 2005. URL accessed on 2009-01-23.
  • (2000) Insects and Spiders, New York: St. Remy Media Inc. / Discovery Books.
  • Freeman, Scott (2003). Biological Science, Prentice-Hall.
  • Hillyard, Paul (1994). The Book of Spiders, 47–50, New York: Random House, Inc..
  • Hillyard, Paul (1994). The Book of the Spiders, 22–35, New York: Avon Books.
  • Martin, Louise (1988). Black Widow Spiders, 18–20, Rourke Enterprises, Inc..
  • Preston-Malfham, Ken (1998). Spiders, Edison, New Jersey: Chartwell Books.
  • "Arthropod". Microsoft Encarta Online Encyclopedia. (2004). 



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